Eyes Give 360° Vision

Biological Strategy

Chameleons

AskNature Team

Image: Martin Fisch / Flickr.com /

Sense Motion

Perceiving motion is important for a living system to sense where it is in relation to a moving environment, which is critical in locating resources or wayfinding. This applies whether the environment itself is in motion (such as water movement coming from a nearby fish) or the living system is moving within a stationary environment (such as a bird flying through the air). Because motion dampens over distance and the cost of missing those motion signals is high, living systems must be quite sensitive to these signals. For example, fast-flying big brown bats have microscopic, stiff, domed hairs on their wing membranes that act as a sensor array to monitor flight speed and airflow conditions.

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Sense Light (Visible Spectrum) From the Environment

Living systems constantly receive signals from their environment that help them survive. Light (in the visible spectrum) can come from other living systems (such as fireflies) or from non-living sources (such as the sun). Survival often depends on sensing and responding to challenges like low light conditions or light that has been altered in some way. Because basic survival is at stake, living systems must excel at meeting those challenges. A well-known phenomenon is how water bends light. A stork trying to catch a fish underwater can compensate for this bending effect so that when it strikes at the fish, it has a good chance of catching it.

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Modify Position

Many resources that living systems require for survival and reproduction constantly change in quantity, quality, and location. The same is true of the threats that face living systems. As a result, living systems have strategies to maintain access to shifting resources and to avoid changing threats by adjusting their location or orientation. Some living systems modify their position by moving from one location to another. For those that can’t change location, such as trees, they modify position by shifting in place. An example of an organism that does both is the chameleon. This creature can move from place to place to find food or escape predators. But it also can stay in one place and rotate its eyes to provide a 360-degree view so that it can hunt without frightening its prey.

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Capture, Absorb, or Filter Organisms

Many living systems must secure organisms for food. But just as one living system must capture its prey to survive, its prey must escape to survive. This results in capture and avoidance strategies that include trickery, speed, poisons, constructed traps, and more. For example, a carnivorous plant called the pitcher plant has leaves formed into a tube that collect water. Long, slippery hairs within the tube face downward. When insects enter the tube seeking nectar, they lose their footing and slide inside, unable to climb out and escape being eaten and digested by the plant.

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Reptiles

Class Reptilia (“creeping”): Lizards, crocodiles, turtles, snakes

Reptiles retain some of the key characteristics that first enabled vertebrates to live permanently on land. They have dry skin covered in scales made of keratin that help prevent water loss. (Amphibians still need to stick close to water to keep their skin moist.) Many reptiles reproduce by laying eggs that are watertight and have a yolk, so eggs can develop on land while staying moist and nourished. Reptiles are also ectotherms, meaning they don’t produce their own body heat. This means they burn through energy much more slowly than “warm blooded” creatures of the same size. It also means that they need to keep warm to keep active, which is why you might see them spending a seemingly inordinate amount of time simply basking in the sun.

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The eyes of the chameleon provide 360-degree vision due to unique eye anatomy and an ability to transition between monocular and binocular vision.

Chameleons have a distinctive visual system that enables them to see their environment in almost 360 degrees (180 degrees horizontally and +/-90 degrees vertically). They do this in two ways. The first is with anatomical specializations that enable the eyes to rotate with a high degree of freedom. The second is the chameleon’s ability to transition between monocular and binocular vision, meaning they can view objects with either eye independently, or with both eyes together.

Several anatomical features enable chameleons to rotate their eyes to such a high degree. The eyes are located on opposite sides of the head, providing a view to the sides and behind or toward the front. Internally, the eye balls are mounted in twin conical turrets (like two upside down ice cream cones). Without a deep orbital socket to keep the eye from falling out (as in humans), the chameleon has evolved a thick, muscular lid. This lid surrounds each eye turret, leaving only the pupil exposed. This provides a “safety net” that enables the eye to bulge out of the conical turret. Without the restriction of a deep orbital socket, each eye can rotate nearly 180 degrees, giving a much wider range of vision than animals whose eyes are secured in socket structures.

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The ability to transition between monocular and binocular vision also enables the chameleon to view objects panoramically. While searching for prey, the chameleon uses monocular vision, with each eye functioning independently of the other. The eye movements–or saccades–are referred to as ‘uncoupled’ when functioning this way. Two separate bundles of nerves control the musculature of the eyes, and two separate images are sent to the brain. Once the chameleon spots its prey, the saccades synchronize, in a process called “coupling,” and both eyes lock on the object. For coupling to occur, visual signals are first sent to the brain through two non-coupled neural bundles. The brain reads these signals, and the eye that has spotted the prey sends stronger electrical impulses to the brain than the eye still searching for the target. The neuron from the eye that does not see the prey syncs with the one that does, forming a larger neural bundle. Once the eye movements are synchronized, the eyes fix on the object and only the head rotates.

The chameleon’s ability to switch freely between synchronous and uncoupled saccadic eye movement is like having two movies playing in your head, and if you wanted to only watch one, you could. This enables the chameleon to operate as both a binocular and monocular organism in a remarkably efficient way for protection, food gathering, and reflexes.

This summary was contributed by Allie Miller.

Last Updated September 14, 2016

References

“Perhaps the strangest of animal eyes belong to the chameleon. They are mounted in twin conical turrets and can move independently of each other, giving the chameleon the ability to see all round itself when seeking prey, and binocular vision in front when it is preparing to strike with its long, sticky tongue.” (Foy and Oxford Scientific Films 1982:127)

Book

The grand design: Form and colour in animals

26/02/1983 | Sally Foy

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Reference

“…[T]he chameleon has [sic] highly independent eye movements, with a pattern of alternation of saccadic eye movements between each eye. In the Chameleon, the number of saccades in one eye before the switch to the other, is usually one, but three or four could be observed (see Supplementary material). Photoretinoscopy measurements show that the alternation of each eye also extends to accommodation, which uses visual feedback to control retinal image focus only in the ‘active’ eye. These observations support the interpretation that attention is alternating from eye to eye along with the oculomotor switch…[I]t seems likely that the switching mechanism helps eliminate the ambiguity that would result if both eyes were to simultaneously acquire different prey targets.” (Pettigrew et al. 1999:421-422)

Journal article

Convergence of specialised behaviour, eye movements and visual optics in the sandlance (Teleostei) and the chameleon (Reptilia)

Current Biology | 25/07/2002 | John D. Pettigrew, Shaun P. Collin, Matthias Ott

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Reference

“When no prey item was fixated, disconjugate saccades were observed which was in accordance with earlier observations in chameleons. During prey tracking the chameleons switched to a different oculomotor behaviour and pursued the moving prey with synchronous saccades. At higher target velocities, the tracking movement of the head was also saccadic and was synchronised with the two eyes.” (Ott 2001:173)

Journal article

Chameleons have independent eye movements but synchronise both eyes during saccadic prey tracking

Experimental Brain Research | 12/02/2003 | Matthias Ott

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Reference

“The scleral cartilage (ring) is present and in chamaeleon is formed by 11 scleral ossicles creating a conical form. It is confined to the orbital hemisphere in the scleral layer of eye with the cornea extending out of center. This scleral ossicle is coated with fine muscle fibers from the M. depressor palpebralis inferior of the eyelid just below the surface of the skin. This eyelid depressor muscle extends from the rim of the eyelid ventromedially around the eye in a thin sheet to the ventral and medial aspect of the orbit where it originated on the palatine and interorbital membrane.” (Tolley and Herrel 2013: 44)

Book

The Biology of Chameleons

16/11/2013

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